Imidazoles and simple imidazole derivatives are being used more often as hardening agents for epoxy resins. They provide long pot life, high heat distortion temperatures, economical performance based on low PHR requirements and are less toxic than amines. They are useful accelerators for anhydride cure and bisphenol-A cure of epoxy resins. The imidazole ring can be utilized in numerous reactions and the derivatives can be used for specialty epoxies. For example, delayed action can be obtained by acylating imidazoles with polychlorinated benzoyl chloride. See U.S. Pat. No. 4,436,892. ##STR1##
Imidazoles are characterized by their essentially aromatic character, high boiling points, and good stability to oxidizing and reducing agents. They undergo typical aromatic reactions such as nitration, chlorination and diazo coupling. The 2-alkyl and 2-alkenyl imidazoles show a strong detergent and emulsifier action in petroleum oils and they protect metals in contact with such oils.
The long chain 2-alkyl imidazoles are surface active agents and the corresponding quaternary compounds show typical germicidal properties of cationic surfactants. Some other uses include fabric softeners (Europa Chemie Apr. 30, 1983, p. 195), antifungal agents (World Pharmaceutical News, Apr. 20, 1981, p. 15), photosensitive product reagents for the preparation of urethane resins, catalysts for the preparation of polyesters and polyurethanes, plastic additives, anticonvulsant drugs, and antimicrobial agents. Certain imidazoles are effective in the control of anthrenus flavipies, an insect pest of wool.
Early synthesis of imidazoles involved the reaction of 1,2-dicarbonyl compounds with ammonia and aldehydes to produce low yields according to the equation: ##STR2##
The yields could be increased if the reaction was carried out in organic acid with ammonium acetate. Radziszewski, R. Ber, 15, 2706 (1882).
Imidazole oxalate, fumarate, adipate, phthalate and 4-methylimidazole, 4,5-dimethylimidazole and 2-isopropylimidazole have been made from the .alpha.,.beta.-dicarbonyl compounds, as demonstrated is U.S. Pat. No. 3,715,365.
Werdenhazen, R., and Rienacker, H., Ber. 72, 57 (1939) demonstrated the production of imidazoles from .alpha.-hydroxyketones under the influence of ammoniacal cupric acetate and aldehydes represented by the following: ##STR3##
In "New Methods of Preparative Organic Chemistry", Vol. 3, p. 241, Academic Press. N.Y., 1964, H. Budereck, et al. describe another method involving formamide synthesis according to the equation: ##STR4##
Some 2-methylimidazoles have been prepared using acetamides; however, yields were reduced when amides other than formamide were used.
One of the more attractive methods for producing imidazoles is discussed in U.S. Pat. No. 2,891,966. This involved the reaction of a 1,2-diamine with carboxylic acids. For instance, ethylenediamine was admixed with a slight excess of acetic acid, permitting an exothermic rise which dissolved the reagents. The homogeneous solution was cooled to about 100.degree. C., then charged through a continuous reactor equipped with a preheater section (a reactor section filled with platinum). Then the reactor was heated to 430.degree. C. and hydrogen added. The vaporous reaction effluent was condensed to obtain crystalline 2-methyl imidazole.
A French patent describes the purification of 2-methyl imidazole by codistilling it with 1- or 2-methylnaphthalene and then washing with pentane or toluene. See French P. 1,362,689 (1964).
In other work, the diamine is converted to 2-alkyl imidazoline, and then dehydrogenated to the corresponding imidazole compound by dehydrogenation over a nickel catalyst. For example see U.S. Pat. No. 2,399,601 and U.S. Pat. No. 2,404,299 on the preparation of imidazoles by heating imidazolines with Raney nickel catalysts. The reactions were carried out at 225.degree.-235.degree. C. The yields were not reported in some cases or varied widely. In these patents the intermediate imidazoline had to be isolated.
H. A. Green, of Air Products has demonstrated that 1,2-diamines can be reacted with aldehydes and then heated over a platinum-alumina catalyst at 370.degree. C. to give imidazoles. In the case of ethylenediamine and propionaldehyde a 56% yield of 2-ethyl imidazole was obtained. See U.S. Pat. No. 3,037,028, May 29, 1962.
In U.S. Pat. No. 3,037,028, using another vapor phase reaction, Green demonstrated that imidazole could be obtained from ethylenediamine and formamide using a large volume of hydrogen. See also U.S. Pat. No. 3,255,200. The catalyst used was platinum-on-alumina and alumina or cobalt molybdate were shown to be ineffective. Treating ethylenediamine with methyl formate at 25.degree.-30.degree. C. gave 98.5% diformyl derivate.
In Ger. Offen. DE 3,009,605, diformyl derivate was passed with nitrogen over 6:14 NiO:MoO.sub.3 at 400.degree. C. to give 65.7% imidazole with 99.3% conversion. The yield remained constant after 250 hours use of the catalyst.
Imidazolines can be dehydrogenated to imidazoles at 250.degree.-500.degree. C. over MoO.sub.3 and NiO and/or CoO and Al.sub.2 O.sub.3, SiO.sub.2 and/or alkaline silicate catalysts. See DE 3,009,631. This reaction has been used to make a variety of 2-alkylimidazoles substituted with long chain fatty acids. As noted, the minimum temperature requirement is 250.degree. C.
Another route to imidazoles involved the reaction of a nitrile with a diamine over a copper salt to give imidazolines which were then dehydrogenated over an aluminum-zinc oxide catalyst to give imidazoles, involving two distinct steps. See DE 3,236,598-A to BASF.
In U.S. Pat. No. 4,409,389, hexamethylene-tetramine was reacted with formamide at 140.degree. C. to give a bis-formamide. The bis-formamide with dicarbonyl compounds and 2 moles of mineral acid yielded imidazole acid salts. ##STR5##
In Ger. Offen. 1,952,991, imidazole was made in 53% yield by passing a solution of ethylenediamine and formic acid over a Cd-Cu chromite catalyst at 480.degree. C. This method required quite high temperatures and the yield was very moderate.
Imidazolines have been dehydrogenated with sulfur and manganese dioxide.
In copending application Ser. No. 284,884 there is described the preparation of imidazoles by the dehydrogenation of imidazolines over a nickel catalyst in combination with chromium and/or copper. ##STR6##
It is noted that most methods for producing imidazoles which are found in the art require higher temperatures and the yields reported are not as high as would be desirable. Some require catalysts which are expensive and would be a deterrent to commercial use. Most procedures require several steps, including isolation of an intermediate imidazoline and subsequent dehydrogenation to the imidazole.
It would be a substantial advance in the art if high yields of imidazoles could be prepared from readily available reactants such as carboxylic acids and diamines without the need to isolate the imidazoline intermediate. Such a process would be extremely attractive commercially.